A toner for use as a developing powder for photocopiers, laser printers and other machines in the field of electrophotography often includes an external additive to impart some flowability to the toner, improve its charging efficiency, control the quantity of charge stored, and for other purposes. Silica is generally used as such an external additive.
Recently, photocopiers and printers have been increasingly required to operate at even higher speeds, have a further reduced overall size, even higher color reproducibility, and enhanced image quality or resolution, and achieve other improvements. The toner for use in those high-performance copy machines and printers has also been designed and developed to meet such a growing demand.
Among other things, maintaining good stabilized image quality for a long time is one of essential requirements in the field of electrophotography. To achieve this, the properties of silica, generally used as an external additive to a toner, have been studied from various angles. See, for example, Patent Documents 1, 2, and 3.
Silica is present as an agglomerate with a size on the order of a few ten μm to several hundred μm. This agglomerate is formed as a physical agglomerate of a plurality of agglomerated primary particles, each of which has been formed as an agglomerate, via chemical bonds, of a plurality of primary particles, each having a particle size on the order of a few nanometers to several ten nanometers. Major roles to be played by silica in a toner include imparting some flowability to the toner and stabilizing its charging characteristic. To fulfill these roles, silica needs to undergo an appropriate surface treatment, not just have the size and properties of those agglomerated silica particles controlled properly. Patent Documents 1-3 disclose silica subjected to a surface treatment with silicone oil.